ISLAMABAD – Suspected militants attacked and set fire to at least 20 tankers carrying oil for NATO and U.S. troops in Afghanistan on Monday, the third such strike inside Pakistan in as many days, police said.

The attack not far from the capital Islamabad took place on a supply line that has been stalled because of a temporary border closing imposed by Pakistani authorities to protest a NATO helicopter attack that killed three Pakistan troops last week.

It will raise the stakes in the closure, which has exacerbated tensions between Washington and Islamabad but has been welcomed by Islamist groups opposed to Pakistan's support of the U.S.-led war in Afghanistan.

Police officer Umer Hayat said three people were killed and blamed Monday's attack on "terrorists."

The attackers opened fire on trucks that were parked at a poorly guarded terminal before setting them afire, he and other officers said.

The trucks were en route or waiting to travel to the Torkham border crossing along the fabled Khyber Pass, which is used to bring fuel, military vehicles, spare parts, clothing and other non-lethal supplies for foreign troops in Afghanistan. Pakistan's other main route into landlocked Afghanistan, in Chaman in the southwest, has remained open.

While NATO and the United States have alternative supply routes into Afghanistan, the Pakistani ones are the cheapest and most convenient. Most of the coalition's non-lethal supplies are transported over Pakistani soil after being unloaded at docks in Karachi, a port city in the south.

On Friday, a day after the closure of the Khyber Pass route to NATO and US traffic, there were two attacks on oil tankers headed to the country. The Pakistani Taliban claimed responsibility for at least one of them, and vowed to launch more.

Striking now gains them more media attention than normal and adds to unease between Pakistan and the United States.

The convoys take several days to reach the border after setting off from Karachi and make frequent stops. They receive little or no protection outside the frontier region and are indistinguishable from ordinary trucks and tankers that ply Pakistani roads.

Over the past two years they often have been attacked by militants, mostly in the northwestern border region where militants are strongest.

Attacks on convoys in Pakistan give militants a propaganda victory, but coalition officials say they do not result in shortages in Afghanistan. Hundreds of trucks cross into Afghanistan each day.

Some attacks are believed to be the work of criminals, who can sell much of the vehicles, clothes and other goods they carry. Officials have alleged truck owners may be behind some of the incidents, perhaps to claim insurance fraudulently.

Earlier Sunday, Pakistan's ambassador to the U.S., Husain Haqqani, said on CNN's State of the Union program that he did "not expect this blockade to continue for too long."

Asked whether the route could be opened within the next week, he said "I think it will happen in less than that duration."

U.S. officials are also predicting the route will not stay closed for long.

Analysts have said that the relationship between Pakistan and the United States is too important for both nations for this incident to derail ties.